Here's why:
* Solids: Molecules in solids are tightly packed together and have strong intermolecular forces. They vibrate in place but don't have enough energy to move freely.
* Liquids: Molecules in liquids are closer together than in gases but further apart than in solids. They have more freedom to move around, so they move faster than molecules in solids.
* Gases: Molecules in gases are far apart and move very quickly. They collide with each other and the walls of their container.
So, the slower the molecules move, the more tightly packed they are, and the more difficult it is for them to move around. This is why solids have a fixed shape and volume, while liquids have a fixed volume but not a fixed shape, and gases have neither a fixed shape nor volume.
